For obvious safety reasons, it is now the practice to provide vertically opening closure screens with retaining devices seeking to avoid falling of the shutter in the case of malfunction of an element of the carrying system. Moreover, in certain countries, the standards in force require providing such screens with a safety device.
As to screens provided with springs for compensating the shutter, mounted about a fixed tube, these springs are major elements adapted to undergo malfunction, and the most often present system consists in designing compensation springs integrating their own safety system in case of breakage. However, this solution has principally two drawbacks. Thus, and in the first instance, it gives rise to increase of cost of the screens because of the multiplication of the number of safety devices equal to the number of springs, and moreover it requires replacing the spring provided with its safety device upon breakage of this latter. Still further, upon breakage of a compensating spring, the shutter is blocked in the position in which it is located upon this breakage. However, the compensating springs are often disposed within the roller of the shutter and are thus not directly accessible unless the shutter is entirely taken down. In these conditions, the changing of the broken compensation spring is almost always a hazardous operation requiring frequent damage to the shutter to gain access to said spring.
As to the closure screens mounted about a rotatable tube, there exist at present a plurality of safety systems designed according to the same principle and comprising generally a member in the form of a disc or rotor adapted to be secured to the rotatable tube so as to be driven in rotation with this latter, at least one abutment member mounted pivotally on the rotor about an axis parallel to the axis of the rotatable tube, and a fixed member or stator of generally annular shape, disposed about the rotor and provided with an internal peripheral surface having recesses for blocking the free end of the abutment member.
The principle of all these safety devices consists in using the centrifugal force exerted on each pivotal abutment member, to bring this latter into a blocking position of the shutter when this centrifugal force becomes too great, corresponding to an excessive abnormal speed of unrolling of the screen.
In practice, such safety devices are quite satisfactory when they are designed to be used with closure screens of relatively low weight such as for example used for constructions in habitations. By contrast, they have drawbacks when the closure screens, for example those used for industrial buildings, have relatively great size and hence weight.
Thus, by their design, these safety devices comprise abutment members which are subjected to a phenomenon of rebound during blockage in rotation of the screen, which determines whether their effectiveness can be guaranteed.
However, in practice, overcoming this rebound phenomenon is a problem difficult to solve, as will be seen for example from a consideration of the safety device of the conventional type described in U.S. Pat. No. 5,494,093, in which the peripheral surface of the stator comprises inclined throats toward which the end of the abutment member is guided by ramps provided on said peripheral surface.
According to this design, the rebound phenomenon is avoided thanks to the conjugated shape of the end of the abutment member and the throats which thus permit obtaining wedging of said abutment member within one of the throats.
However, the wedging effort being proportional to the weight of the shutters, the use of such a safety device with shutters of relatively high weight leads either to a blockage with a great wedging effect of the abutment member within one throat, or an inelastic deformation of the stator.
In the first hypothesis, it is practically impossible to unwedge the abutment member without direct access to the safety device, and this operation requires demounting of the drive means of the rotatable shaft so as to gain access to this safety device, then dismounting said safety device which can be delicate because the latter is blocked. Moreover, during this demounting, it is frequently true that the device will be damaged, which requires its replacement.
In the second hypothesis, the stator is deformed in a residual manner and the ultimate operation of the safety device is no longer guaranteed, such that this latter must in this case also be replaced.
Another solution, described in German patent DE 24 33 544, and also applicable to screens with a rotatable tube, consists in driving in rotation with the tube counterweights adapted to pivot about axes parallel to the axis of said tube, such that one of said counterweights will become engaged in abutment teeth of a fixed plate when the centrifugal force exerted on this latter exceeds a given value determined by resilient means, in a position of blockage in which it is maintained automatically by a pawl.
According to this device, the rebound phenomenon is thus mastered by means of a pawl which ensures maintenance of the counterweight in its blocking position. However, this solution leads to the same drawback as that recited for shutters with compensation springs, namely that the shutter is blocked in the position in which it is located upon breakage of a motor element.